Helically-strapped multifilar helices



Feb. 7, 1961 D. G. DOW

HELIcALLY-STRAPPED MULTIFILAR HELICES Filed July 23, 1959 INVENTOR.DAN/EL 6. DOW '8? 7M 3T 51 l United States Patent HELICALLY-STRAPPEDMULTIFILAR HELICES Daniel G. Dow, Altadena, Calif., assignor, by mesneassignments, to the United States of America as represented by theSecremry of the Navy.

Filed July 23, 1959, Ser. No. 829,167

4 Claims. c1. SIS-3.6)

This invention relates to a slow-wave structure for traveling-waveamplifier tubes, and more particularly to helically-strapped multifilarhelices. I

An object of the invention is to provide a traveling-wave tube whichwill have a large 3-db bandwidth and which will be very nearly free fromself-oscillation caused by beam interaction with a backward spaceharmonic of the structure having a velocity very nearly synchronous withthe electron beam.

By the aforementioned description of bandwidth is meant to those skilledin the art that ratio of the frequencies at the 3-clb down points on theresponse curve ranges from 1.5:1 to 2:1 or more.

Other objects and many of the attendant advantages of this inventionwill be readily appreciated as the same becomes better understood byreference to the following detailed description when considered inconnection with the accompanying drawings wherein:

Fig. l is a more or less schematic representation of a traveling-wavetube constituting one preferred embodiment of the invention;

Fig. 2 is an enlarged axonometric view of a portion of the slow-wavestructure removed from Fig. 1; and

Fig. 3 is a cross-section taken along the lines 36 in Fig. 1.

Reference is made to the drawing. The preferred illustrated embodimentof the slow-wave structure of this in-- vention is a multifilar helixshown as a bifilar helix comprised of two separate helices 2 and 4.These two separate helices constitute the main helix, of radius a, beingformed of two identical helices spaced equally along a common axis.Although the drawing shows, as noted, a bifilar helix (two helix array)the invention comprehends the use of more than two identical helices toconstitute the main helix. Auxiliary helices 6 provide a firmelectrically conducting contact with the main helices wherever they meetand serve to electrically join the separate helices of the main helix inconducting relation. The auxiliary helices can also be multiple-wirehelices, but it is satisfactory to make them as illustrated, in the formof simple, single or monofilar helices. The auxiliary helices areconstructed in such a way that the average phase velocity of anelectromagnetic wave in the fundamental helix mode, along either one ofthem by itself, is approximately the same as the velocity of the similarmode along the separate helices constituting the main helix, whenconsidered by themselves.

The illustrated helix is shown in Fig. 1 embodied in a conventionaltraveling-wave tube, generally indicated at 8 which includes an electrongun 10, a collector 12, input terminals 14, and output terminals 16. Theelectron beam travels generally along the axis of the main helix. Thehelically-strapped multifilar helix slow-wave structure can be supportedby simply resting against the glass envelope of the tube, if desired.

This type of support oifers an advantage over certain other slow-wavestructures in that it requires no obstructing interior supports withinthe main helix and hence can h zsitiit Patented Feb. 7, lfifil beconveniently used with a single electron beam inside the structure, thussimplifying a number of constructional problems.

Although there can be any number of auxiliary helices, the most usefulsituation occurs when the number of auxiliary helices is the same as thenumber of helices in the central portion or main helix of the structure.These auxiliary helices are spaced evenly around the circumference ofthe main array.

The cross-sectional form of the conductors constituting the varioushelices can be that of a tape, as in the illustrated embodiment, or canbe of any other convenient shape.

The relative diameters of the main helices and the auxiliary helices mayvary. The larger the auxiliary helices, relative to the main helices,the less chance there will be of backward-wave oscillation, but therewill be some simultaneous lowering of interaction efiiciency.

The individual phase velocities of the various helices can be modified,depending on various non-ideal properties of a particular system, suchas dielectric loading, and electronic loading, which may be frequencydependent.

Operation An understanding of the operation of the helicallystrappedhelix of this invention can perhaps best be achieved by considering anunstrapped multifilar helix in the form of a bifilar helix.

The unstrapped bifilar helix has an infinity of possible modes ofpropagation, two of which are important here. The so-called even mode ischaracterized by fields at the two helices being in phase with eachother. It has a forward-traveling space harmonic which is useful forWide band forward-wave amplification and a moderately strong 2 backwardspace harmonic which may cause oscillations at extremely high currents.An odd order or push-pull mode also exists for which the fields at thetwo helices are out of phase at any cross-section. This mode has a verystrong backward space harmonic that will generally cause undesiredoscillations necessary for forward amplification.

The presentinvention utilizes the very strong coupling between the innerand outer helices to distort or suppress completely the odd mode. Sincethe even mode synchronizes approximately with the auxiliary helices,there will be negligible effect on the propagation velocity of this mode(which is the one desired for forward-wave amplification). The odd mode,however, has no counterpart on the auxiliary helices, since they have atighter spacing, in general, and thus a different set of spaceharmonics, including none which are close to those of the odd mode ofthe multifilar main helix. Thus the odd mode will be severely distortedby the auxiliary helices.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described.

What is claimed is:

1'. A slow-wave structure for use in traveling-wave tubes comprising amultifilar helix including a plurality of separate parallel intertwinedhelices, each separate helix having the same constant pitch, the sameconstant pitch angle,

helix and in firm electrically-conducting contact with the said separatehelices wherever they meet, the auxiliary helices being tangent to thesaid separate helices and having their axes parallel to the axis of themultifilar helix.

2. The slow-wave structure of claim 1 wherein the mul: tifilar helix isa 'oifilar helix.

3. The slow-wave structure of claim 1 wherein the number of auxiliaryhelices equals the number of separate helices constituting themultifilar helix.

4. In a traveling-Wave tube, an electron gun and a collector forelectrons spaced apart in said tube for forming an electron beam, and aslow-wave structure; said .slow-wave structure comprising a multifilarhelix positioned to surround said electron beam and including aplurality of separate parallel intertwined helices, each separate helixhaving the same constant pitch, the same constant pitch angle, and sameconstant mean radius, and lying along a common axis; said separatehelices being spaced from each other in such a manner that the distancemeasured in a direction parallel to the axis of the multifilar helixbetween a point on one turn of the multifilar helix and thecorresponding point on the next adjacent turn of the multifilar helix isthe same for every such pair of turns of the multifilar helix; and aplurality of auxiliary conducting helices, spaced evenly around thecircumference of said multifilar helix and in firmelectrically-conducting contact With the said separate helices whereverthey meet, the auxiliary helices being tangent to the said separatehelices and having their axes parallel to the axis of the multifilarhelix.

References Cited in the file of this patent UNITED STATES PATENTS TienJan. 12, 1960

